A switching-mode power supply (SMPS) can be used to drive a string of LEDs. The SMPS often includes a full-wave rectifier circuit for rectifying an AC input voltage into a DC voltage. The DC voltage provides input current to the LEDs. The SMPS can include PFC circuitry that controls the input current so that the input current waveform is in phase with the waveform of the AC input voltage (e.g., a sine wave). For a good power factor, the input current waveform will have the same shape and phase as the AC input voltage waveform, but will vary in magnitude or Root Mean Square (RMS) value. A good power factor can help efficient delivery of electrical power from the AC input voltage source to the LEDs.
Conventional SMPS circuits for LED drivers include one or two stage PFC converters, or a primary side driver system. Two stage PFC converters add cost and have lower efficiency due to the second stage of conversion. Single stage PFC converters require large electrolytic capacitors, which is bulky and unreliable and can shorten the life of the SMPS system. Single stage PFC converters are also not suitable for LED dimming applications. Primary side driver systems are susceptible to color shifting caused by changing current in the LEDs. In addition, since the system is off for a substantial portion of time the power factor is deteriorated.